Method and apparatus for pulse generation and adaptive pulse generation for optical communications

ABSTRACT

One embodiment of the invention relates to producing optical pulses for use on a transmission link. A light source is configured to produce an optical signal. A pulse generator is coupled to the light source. The pulse generator is configured to receive, for a first channel, the optical signal and a clock signal. The pulse generator is also configured to modify the optical signal based on the clock signal to produce an optical pulse having a predetermined pulse shape. The clock signal is associated with the predetermined pulse shape. The predetermined pulse shape being based on a transmission characteristic of the transmission link.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/215,036 filed Aug. 9, 2002 now U.S. Pat. No. 7,266,307. Thisapplication claims priority to U.S. patent application Ser. No.10/084,057, entitled “Method and System for mitigating nonlineartransmission impairments in fiber-optic communications systems,” filedon Feb. 28, 2002, which claims priority to No. 60/352,991, entitled“Optical communication system and method,” filed on Feb. 1, 2002; boththe entirety of which are incorporated herein by reference. Thisapplication is also continuation-in-part of the patent application Ser.No. 10/173,579 “Light source for generating output signal having evenlyspaced apart frequencies,” filed on Jun. 18, 2002, the entirety of whichis incorporated herein by reference.

BACKGROUND

The invention relates generally to optical communications such as asystem and method for optical communications with data rates of at least10 Gbit/s.

Many of optical communication systems with data rates of at least 10Gbit/s are based on a return-to-zero (RZ) format for data transmission.Such known optical communication systems typically relate to generatingan optimal pulse shape or to evaluating the pulse shape once generated.

For example, adaptive pulse shaping in free space and in the 850 nmspectral range had been disclosed in D. Yelin et al., “Adaptivefemtosecond pulse compression,” Opt. Letters, 1997, v. 22, #23, pp.1793-1795. The input pulse arrives from a comb generator (in this case,a mode-locked laser), and is spatially separated into its spectralcomponent by a grating. A lens maps each wavelength group onto aseparate pixel of a computer-controlled phase modulator. A second lensand grating recombine the components back together to form a controlledoutput pulse. A doubler crystal samples the output pulse. This crystalproduces pulses that are higher the shorter the pulse is. By using anappropriate algorithm, a computer can adapt the phases of the differentspectral components of the input pulses so that for each type of inputpulse, the shortest pulse possible should be produced by the device. Theshortest pulse is considered as the most optimal for the communicationsystem.

An alternative but analogous method was disclosed in K. Kitayama et al.“Optical pulse train synthesis of arbitrary waveform usingweight/phase-programmable 32-tapped delay line waveguide filter,”Proceedings of OFC-2001, paper WY3-1. Similar to the Yelin system, thepulse source in Kitayama is a comb generator, but the pulse shaping isdone through a parallel series of delay lines and attenuators.

These known devices, however, suffer several shortcomings. For example,these known devices are quite complex from the technical point of view.In addition, because these known devices typically relate to generatingan optimal pulse shape or to evaluating the pulse shape once generated,such devices are bulky, expensive and not appropriate for use incommercial systems.

Moreover, in real optical communication system, either terrestrial orundersea, the fiber conditions and multiple component operations changein time. Therefore, the optimal pulse shape is different for the everyparticular time interval. The best performance of the pulse generator orpulse shaper should include a closed loop to correct adaptively thechanging conditions. An adaptive approach for the pulse shaping in fibercommunication has been developed by a number of research groups (see,for example, F. G. Omenetto, M. D. Moores, B. P. Luce, D. H. Reitze andA. J. Taylor “Femtosecond pulse delivery through single-mode opticalfiber with adaptive pulse shaping,” Proceedings CLEO′2001, pp. 234-235).Indeed, such an approach can provide a mechanism to overcome multiplelimitations associated with nonlinear effects and provides anopportunity to synthesize pulses that are self-correcting for higherorder nonlinear effects when being launched in the fiber.

As discussed below, in the present invention, a device is described thatcan be implemented in real RZ communication systems and can provide anumber of advantages from the point of view of chromatic dispersionreduction and nonlinear effects mitigation. This results in animprovement of the communication link figure of merit:cost/(capacity*distance). The described device provides new technicalsolutions in the pulse formation and in the pulse shape evaluationtogether with adaptive shaping in time.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to producing optical pulses foruse on a transmission link. A light source is configured to produce anoptical signal. A pulse generator is coupled to the light source. Thepulse generator is configured to receive, for a first channel, theoptical signal and a clock signal. The pulse generator is alsoconfigured to modify the optical signal based on the signal to producean optical pulse having a predetermined pulse shape. The signal isassociated with the predetermined pulse shape. The predetermined pulseshape being based on a transmission characteristic of the transmissionlink.

Another embodiment of the invention relates to the generation of pulseshaving a pre-determined shape using amplitude or phase modulation bygenerating at least two signals at strong side harmonic frequencies andcombining them to create the pulse.

Another embodiment of the invention relates to the method and device forgenerating pulses having a pre-determined shape and pre-chirp using acombination of amplitude or phase modulators and a slow phase shifter.The slow phase shifter produces a chirp by introducing a relative phaseshift between the carrier and the side harmonics.

Another embodiment of the invention relates to the method and device forgenerating pulses having a pre-determined pulse shape based oncomb-generator that produces light with a set of frequencies evenlyspaced apart.

Another embodiment of the invention relates to measuring an opticalpulse shape after being transmitted on a communication link. Aphotodiode is configured to receive an optical pulse having a firstspectral component, a second spectral component and a third spectralcomponent. The second spectral component and the third spectralcomponent are based on a clock frequency. The photodiode is configuredto send a first signal having an amplitude and a spectral component withthe clock frequency. A filter is coupled to the photodiode. The filterhas a spectral response associated with the clock frequency. A detectoris coupled to the filter. The detector is configured to send an errorsignal based on the amplitude of the first signal.

Another embodiment relating to measuring an optical pulse shape includesa dispersion device and a balanced detector. The dispersion device isconfigured to receive a first portion of an optical signal on a firstoptical path and a second portion of the optical signal on a secondoptical path. The dispersion device is further configured to introduce afirst dispersion into the first portion of the optical signal and asecond dispersion into the second optical signal. The first dispersionhas its own amplitude and sign. The second dispersion has its ownamplitude and sign. The amplitude of the first dispersion issubstantially equal to the amplitude of the second dispersion. The signof the first dispersion is opposite of the sign of the seconddispersion. The balanced detector coupled to the dispersion device.

Another embodiment relating to measuring an optical pulse shape is amethod based on autocorrelation. The device includes, for example, anoptical hybrid, delay line and balanced detectors. In this embodiment,as described below, the width of the incoming optical pulse is measurednot in one short measurement but through a series of measurements overmultiple pulses.

Another embodiment of the invention relates to adaptive pulse shaping.An optical signal received from a transmission link is measured. Theoptical signal includes a set of optical pulses having an estimatedpulse width. A first dispersion is introduced into a first portion of anoptical signal on a first optical path. A second dispersion isintroduced into a second portion of the optical signal on a second path.The first dispersion has its own amplitude and sign. The seconddispersion has its own amplitude and sign. The amplitude of the firstdispersion is substantially equal to the amplitude of the seconddispersion. The sign of the first dispersion is opposite of the sign ofthe second dispersion. The first portion of the optical signal isdetected after the introducing of the first dispersion, and the secondportion of the optical signal is detected after the introducing of thesecond dispersion to produce a balanced-detected signal.

Another embodiment of the invention relates to the software ditheringmethod for adaptive pulse shaping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system block diagram of a communication system, accordingto an embodiment of the invention.

FIG. 2 shows a system block diagram of a pulse generator using aMach-Zehnder (MZ) modulator, according to an embodiment of theinvention.

FIG. 3 shows Q-factor as a function of the pulse shape parameter α₂ fortwo different transmission distances of 2000 and 1500 km.

FIGS. 4 and 5 show the normalized amplitude versus time ofdata-modulated signals corresponding those referred to FIG. 3 for the1500 km and 2000 km communication systems, respectively.

FIG. 6 a shows the normalized intensity as a function of time for anexample of quasi-RZ pulses having a linear chirp.

FIG. 6 b shows the frequency versus time for the quasi-RZ pulses shownin FIG. 6 b.

FIG. 7 shows a pulse generator, according to an embodiment of theinvention.

FIG. 8 shows a pulse generator, according to another embodiment of theinvention.

FIG. 9 shows a pulse generator, according to yet another embodiment ofthe invention.

FIG. 10 shows a comb-generator light source and a pulse generator,according to another embodiment of the invention.

FIG. 11 shows a pulse shape detector according to an embodiment of theinvention.

FIG. 12 show the time response (in ps) of the pulse shape detector shownin FIG. 11 for a modulation frequency F=12.5 GHz.

FIG. 13 shows a pulse shape detector based on autocorrelation, accordingto another embodiment of the invention.

FIG. 14 shows a pulse shape detector, according to another embodiment ofthe invention.

FIG. 15 shows a pulse shape detector with single chirped fiber Bragggrating, according to an embodiment of the invention.

FIG. 16 shows a pulse shape detector with single chirped fiber Bragggrating, according to another embodiment of the invention.

FIGS. 17 a through 17 d shows an example of a series of pulse trains asan original distorted pulse shape is adaptively corrected, resultingfrom the method described in connection with FIG. 18.

FIG. 18 shows a flowchart of a method for adaptive shape correction,according to an embodiment of the invention.

FIG. 19 shows the test setup for measuring pulse propagation in a linkwith significant dispersion, according to an embodiment of theinvention.

FIGS. 20 a and 20 b show an example of the output pulse waveform on anoscilloscope based on the test setup of FIG. 22.

FIGS. 21 a and 21 b show an example of the output pulse spectrum on anOSA corresponding to the output pulse waveform shown in FIGS. 20 a and20 b.

FIG. 22 shows a test setup for testing the method for adaptive pulseshape correction described in connection with FIG. 17.

DETAILED DESCRIPTION

FIG. 1 shows a system block diagram of a communication system, accordingto an embodiment of the invention. As shown in FIG. 1, the communicationsystem includes a light source 100, pulse generator 110, communicationlink 10, pulse shape detector 20, computer 130, and adaptive phase andattenuation control 120. The light source 100, pulse generator 110,adaptive phase and attention control 120 and computer 130 are typicallylocated within a transmitter system. The pulse shape detector 20 istypically located within a receiver system. The communication link 10connects the transmitter system and the receiver system.

FIG. 2 shows a system block diagram of a pulse generator using aMach-Zehnder (MZ) modulator, according to an embodiment of theinvention. As shown in FIG. 2, pulse generator 200 includes MZ modulator210, which can be for example an MZ LiNbO₃ modulator. MZ modulator 210receives continuous-wave (CW) light 201 and a clock signal 202 thatincludes a swing voltage and a voltage bias, and produces output opticalpulses 203. The pulse generator 110 shown in FIG. 1 can be embodied bypulse generator 210 of FIG. 2. CW light 201 can be provided to MZmodulator 210, for example, by light source 100 of FIG. 1. The clocksignal 202 can be provided to MZ modulator 210, for example, by adaptivephase and attenuation control 120.

In this embodiment, the temporal form of the pulse shape of outputoptical pulses 203 can be given by:I _(out)=sin[α₁+α₂ cos(2π/T+π)],  (1)

where T is a bit period, t is time, α₁ and α₂ are variable parameters.When such a pulse shape is formed, for example, by passing CW light 201through MZ modulator 210, α₁ is the swing voltage and α₂ is the voltagebias.

The appropriate selection of values for parameters α₁ and α₂ provides apulse shape that is substantially optimal for particular fiber plantsuch as communication link 10 shown in FIG. 1. The term “optimal pulseshape” means a pulse shape that has reduced non-linear penalties,reduced inter-channel interference and reduced chromatic dispersiondistortions of the signal as compared to other possible pulse shapes. Asa result of using substantially optimal shape pulses in a communicationsystem, the signal/noise ratio of the received signal is substantiallymaximized. The value of the signal/noise ratio in a fiber-opticcommunication system is usually described by the term Q-factor.Bit-error-rate (BER) is related to Q-factor asBER=exp(−Q²/2)/(Q(2π)^(1/2)) in the case of single detector. Forexample, a Q-factor value of 6 corresponds to a bit-error rate (BER)value of 10⁻⁹.

A given value of the Q-factor depends, at least in part, on theselection of α₁ and α₂ provided to the MZ modulator 210. FIG. 3 showstwo examples of a particular solution of the Q-factor as a function ofα₂, the voltage bias, according to the embodiment of the MZ modulatorshown in FIG. 2. More specifically, FIG. 3 shows the values of theQ-factor as a function of α₂, the voltage bias, for two long-haul (2000km and 1500 km) optical communication systems using binary phase shiftkeying (BPSK) data modulation. As FIG. 3 illustrates, the value of theQ-factor can be optimized by selecting, among other things, anappropriate value of the voltage bias of the clock signal provided to MZmodulator 210 by adaptive phase and attention control 130. Although notshown in FIG. 3, the selection of the swing voltage also affects thevalue of the Q-factor.

FIGS. 4 and 5 show the normalized amplitude versus time ofdata-modulated signals corresponding those referred to FIG. 3 for the1500 kin and 2000 km communication systems, respectively. As FIGS. 4 and5 show, the optimal pulse shape in this case is quasi-RZ signal.Simulations conducted by the inventors show that about the same valuesof α₁ and α₂ parameters will be appropriate for on-off-keying (OOK) datamodulation format in long haul optical fiber communication. In this caseof the OOK data modulation format, a quasi-RZ signal will also be asubstantially optimal pulse shape. A pulse temporal form based on othersmooth functions such as a Gaussian function and/or polynomials type offunctions may also be used. A higher order Gaussian, such as superGaussian, may also be used.

Other embodiments of the pulse generator are based on the formation ofseveral harmonics of incoming CW light and the appropriate choice of therelative phase and amplitude of these spectral components. Although someof the embodiments described herein use three spectral components, alarger number of spectral components is also possible. Simulationsconducted by the inventors show that the optimal pulse shapes for densewavelength-division multiplexing (WDM) communication systems with phasemodulation of data, quasi-RZ pulse shapes like those shown in FIGS. 4and 5 for particular distances, can be formed by 5 spectral harmonicswith the accuracy of about 2%, and by 3 spectral components with theaccuracy of about 5%. These accuracies can be obtained by theappropriate selection of spectral components' amplitudes and relativephases. Alternatively, a variety of other pulse shapes based on smoothfunctions can be formed.

In another embodiment of the invention, the pulses formed by synthesisof several harmonics are pre-chirped. Pulse pre-chirping is widely usedin fiber communications to compensate for chromatic dispersion ofcommunication line (i.e., chromatic dispersion that occurs within thetransmission fiber during propagation of light signals). In fibercommunication links with compensated chromatic dispersion, some residualdispersion that varies due to environmental changes exists. The pulsepre-chirping can compensate for this residual dispersion. FIG. 6 a showsthe normalized intensity as a function of time for an example ofquasi-RZ pulses having a linear chirp. FIG. 6 b shows the frequencyversus time for the quasi-RZ pulses shown in FIG. 6 a. Three spectralharmonics with appropriate choice of relative amplitudes and phases wereused to obtain this pulse shape and pulse pre-chirp.

In other embodiments of the invention, other types of devices andmethods can be used to achieve a substantially optimal pulse shape andpulse pre-chirp. Unlike many known pre-chirping schemes where the pulseusually requires an additional fast phase modulator, certain embodimentsof the invention described below do not require an additional fast phasemodulator. For example, FIG. 7 shows a pulse generator, according to anembodiment of the invention. Pulse generator 700 includes an inputmodulator 710, an asymmetric MZ interferometer (AMZI) 720, phase shifter730 and an AMZI 740. Input modulator 710 can be, for example, a phase oramplitude modulator. Although not shown in FIG. 7, pulse generator 700can include an amplifier to drive the phase shifter 730.

CW light 701 of frequency f formed by, for example, a distributedfeedback laser (for example, JDS Uniphase, model CQF935/508) (not shownin FIG. 7) is provided to input modulator 710. The input modulator 710also receives a clock signal 703 with a bit rate F. The modulator 710can be, for example, a JDS Uniphase LiNbO₃ MZ modulator model 10020427.The output signal from the modulator 710 includes the carrier and twostrong side spectral components: f−F, f, f+F. The term “strong sidespectral components” refers to spectral components f−F and f+F, whichhave non-trivial amplitudes (i.e., amplitudes above the noise floor).

The signal output from the modulator 710 is provided to AMZI 720, whichacts as a demultiplexer and has, for example, a free spectral range(FSR) equal to 2F. AMZI 720 can be, for example, model M0013NPMFP-DPXAby NTT Electronics. AMZI 720 performs signal demultiplexing and dividesthe received light into different waveguides. Light having the spectralcomponent with the frequency f is coupled into waveguide 732, and thelight having the spectral components with frequencies f+F and f−F iscoupled into waveguide 734.

Phase shifter 730 is coupled to waveguide 732 and receives a phaseadjustment signal 705. Phase shifter 730 (for example, produced by JDSUniphase LiNbO₃ MZ modulator model #10024520) introduces chirp by phasemodulating the light having the spectral component with frequency f.Phase shifter 730 phase modulates the light propagating in waveguide 732based on the phase adjustment signal 705. In this manner, the phaseshifter 730 can introduce a desired chirp into the light having thespectral component with frequency f.

AMZI 740, having corresponding characteristics as AMZI 720, combinestogether the light. More specifically, AMZI 740 combines the lighthaving frequency f, with the light having frequencies f+F and f−F toform output optical pulses 709 having a given pulse shape. In otherwords, similar to AMZI 720, which acts as a demultiplexer to separatethe light having different spectral components into differentwaveguides, AMZI 740 acts as a multiplexer to recombine the light havingdifferent spectral components from these different waveguides to produceoutput optical pulses 709.

FIG. 8 shows a pulse generator, according to another embodiment of theinvention. Pulse generator 800 includes modulator 810, AMZI 820, phaseshifter 830 and balanced loop mirror 840. Although not shown in FIG. 8,pulse generator 800 can include an amplifier to drive the phase shifter830.

Modulator 810 receives CW light 801, for example, from light source 100and receives a modulation signal 803 having a bit rate F. Modulator 810produces light having three spectral components: f, f+F, and f−F. Thelight is then provided to AMZI 820 having, for example, a FSR of 2F.Carrier light having the spectral component with the frequency f iscoupled into waveguide 832, and the light having the spectral componentswith frequencies f+F and f−F is coupled into waveguide 834. Phaseshifter 830 phase modulates the light propagating in waveguide 832 basedon the phase adjustment signal 805. In this manner, the phase shifter830 can introduce a desired chirp into the light having the spectralcomponent with frequency f.

After being modulated by phase shifter 830, the light from waveguide 832and the light from waveguide 834 are reflected by balanced loop mirror840. Thus, pulse generator 800 acts in a bi-directional manner. Insteadof using a second AMZI (analogous to AMZI 740 shown in FIG. 7), AMZI 820is used twice: once to split the light into waveguides 832 and 834, andonce again to combine light returning from waveguide 832 and 834. Thevalue of the phase modulation indicated by phase adjustment signal 805and applied to the phase shifter 830 should be half that of phaseadjustment signal 705 for the phase shifter 730 as shown in FIG. 7. Thisis due to the fact that the light passes through phase shifter 830twice, once in each direction.

FIG. 9 shows a pulse generator, according to yet another embodiment ofthe invention. Pulse generator 900 includes modulator 910, AMZI 920,phase shifter 930 and waveguides 932 and 934. Waveguides 932 and 934each have an end with a high reflection coating. Pulse generator 900 issimilar to pulse generator 800 of FIG. 8 where the balanced loop mirror840 is replaced by the two high reflection coatings on the ends ofwaveguides 932 and 934. Although not shown in FIG. 9, pulse generator900 can include an amplifier to drive the phase shifter 930.

For the embodiments shown in FIGS. 7-9, output optical pulses having adesired pulse shape can be produced without the use of phase-lockedlasers. The shape of a generated pulse is controlled by the drive on thephase modulator (e.g., modulator 710, 810 and 910) while the chirp iscontrolled by the phase shifter (e.g., phase shifter 730, 830 and 930).Experiments conducted by the inventors have achieved a chirp of −0.2GHz/ps. In these experiments, the driving voltage of the phase modulator(e.g., modulator 710, 810 and 910) was V_(π/3) and the phase on thephase shifter (e.g., phase shifter 730, 830 and 930) was approximately0.77. pi.

FIG. 10 shows a comb-generator light source and a pulse generator,according to another embodiment of the invention. The light source 1090includes comb generator 1095. The pulse generator 1000 includesdemultiplexer 1010, phase shifter 1020 and multiplexer 1030. Althoughonly a single phase shifter 1020 and multiplexer 1030 are shown in FIG.10, this shown phase shifter 1020 and multiplexer 1030 arc associatedwith one communication channel. Thus, addition sets of phase shiftersand multiplexers (not shown) can be associated with additionalcommunication channels and included within pulse generator 1000.

In this embodiment, the light source 1090 can include a comb generator1095 producing an output with, for example, evenly spaced apartfrequencies as described, for example, in the U.S. patent applicationentitled “Light Source for generating output signal having equallyspaced apart frequencies” filed on Jun. 18, 2002. Alternatively, otherconfigurations of a comb generator can be used. Three, five or morespectral components of the output of the comb generator 1095 can befiltered from the remaining spectral components to create the pulses ofone communication channel. Another set of spectral components can beused to form the pulses of another communication channel, and so on.Similar to the above-described embodiments of FIGS. 7 through 9, theembodiment shown in FIG. 10 using a comb generator can create theoptical pulses with a substantially optimal shape and pre-chirp.

More specifically, light output from comb generator 1095 isdemultiplexed by demultiplexer 1010. Three (or more) spectral componentsare used to create one communication channel. Phase shifters 1020 createthe desired pulse shape for each communication channel. The parametersof these phase shifters 1020 can be controlled adaptively to follow thechanges in the dispersion map and non-linear properties of thecommunication link (i.e., the optical transmission fiber).

In an alternative embodiment of a pulse generator, a portion of thepulse generator of FIG. 10 can be combined with a portion of the pulsegenerator of FIG. 7, 8 or 9. More specifically, a comb-generator lightsource can be coupled to a demultiplexer having an output channelbandwidth substantially equal to three (or more) spectral component ofthe comb-generator light source. In other words, each optical signaloutput from the demultiplexer has light with at least three spectralcomponents associated with a particular information channel. Eachoptical signal output from the demultiplexer can be received by thefirst AMZI shown in FIG. 7, 8 or 9. This first AMZI can then isolate thelight having one spectral component (e.g., light having the frequencyf). A slow-phase modulator can then modulate the light. The light havingthe one spectral component and light having the remaining spectralcomponents can then be combined to create the desired pulse shape in therespective communication channel.

The practical solutions for the pulse shape formation are not limited tothese described embodiments. Many other embodiments are possible wherethe spectral components of the light are separated so that the lighthaving at least one spectral component can be modified thereby creatinga desired pulse shape.

The pulse generators described above can be used in conjunction with apulse shape detector located at the receiver side of the communicationsystem (e.g., pulse shape detector 20 as shown in FIG. 1). To close theloop and have the subsequently transmitted pulse shape be adapted basedon the dispersion of the link (e.g., communication link 10), an errorsignal can be produced at the transmitter. This error signal should be ameasure of the deviation from the nominal pulse shape due to thetransmission of a pulse through the communication link. This can beaccomplished in several ways such as, for example, measuring the pulsewidth using a standard method like oscilloscope or an autocorelator, andthen modifying the pulse shape based on the difference between thedesired value and the actual value as the error signal.

The difference between the desired pulse shape and the actually receivedpulse shape can be determined in a number of ways. For example, oneembodiment described below uses an autocorrelator approach.Alternatively, the physical properties of the received pulse that are astrong function of the pulse width, such as the pulse's second opticalharmonic or third optical harmonic, can be used. Regarding the secondoptical harmonic, see, for example, D. Yelin, D. Meshulach, and Y.Silberberg, “Adaptive femtosecond pulse compression”, Opt. Letters,1997, v.22, #23, pp. 1793-1795. Regarding the third optical harmonic,see, for example, D. Meshulach, Y. Barad, and Y. Silberberg “Measurementof ultrashort optical pulses by third-harmonic generation”, J. Opt. Soc.Am. B, vol. 14, #4, pp. 2122-2125. The magnitudes of these harmonics canbe used as the error signal. Yet another way is to use a system monitor,like BER data or eye diagrams to produce the error signal.

FIG. 11 shows a pulse shape detector according to an embodiment of theinvention. In this embodiment, the magnitude of the detected microwavesignal is used as a characteristic of the pulse width. Morespecifically, a signal having a clock frequency F (initially used at thepulse generator to modulate the light having a frequency f) is isolatedand its magnitude is measured. This measured magnitude of the signal iscorrelated with the pulse width of the received optical signals.

The incoming optical signal 1101 includes at least three spectralcomponents f_(1−F), f₁, f_(1+F) and is detected by PIN photodiode 1110.The electrical signal 1103 output from PIN photodiode 1110 includes asignal term that corresponds to the intensity of the optical signal 1101at frequency F. Bandpass filter 1120 separates the spectral component atfrequency F of signal 1103 from the remaining spectral components ofsignal 1103 to produce signal 1105. Bandpass filter 1120 can be tuned,for example, to frequency F. The intensity of signal 1105 is detected byRF detector 1130 to produce signal 1107.

FIG. 12 show the time response (in ps) of the pulse shape detector shownin FIG. 11 for a clock frequency F=12.5 GHz. Under real operationalconditions, only the right side of the curve in FIG. 12 is relevantbecause optical pulses shorter than 25 ps are not typical in opticalcommunication systems. Consequently, a unique voltage can be assigned toeach pulse width measured for received optical pulses. Thus, the timeresponse shown in FIG. 12 can be used to define a desired error signal.Note that a modulation frequency of F=25 GHz corresponds to the minimalmeasurable pulse width equal to 12.5 ps.

For embodiments where the incoming optical pulse is phase modulated withdata, the spectral content of the optical pulse becomes more complicatedand can obstruct the proper operation of methods that depend onmeasuring the pulse width. Most communication systems, however, aredesigned so that un-modulated training pulses periodically occur. Forexample, in a 12.5 GHz pulse train, 30 pulses for every 1000 pulses canbe un-modulated training pulses so that only few pulses-per-minute (ppm)will be dedicated to training pulses rather than pulses carrying data.This few number of un-modulated training pulses, however, can besufficient to provide appropriate adaptive dispersion control when inthe right pulse rate and with an effective signal-to-noise ratio.

FIG. 13 shows a pulse shape detector based on autocorrelation, accordingto another embodiment of the invention. In this embodiment, as describedbelow, the width of the incoming optical pulse is measured not in oneshort measurement but through a series of measurements over multiplepulses.

Pulse shape detector 1300 includes polarization controller 1310 splitter(not shown), tunable delay device 1320, optical hybrid 1330, detectors1340, analog-to-digital (A/D) converter 1350 and processor 1360. Thepolarization controller 1310 is coupled to tunable delay device 1320 andoptical hybrid 1330. Tunable delay device 1320 is also coupled tooptical hybrid 1330. Optical hybrid 1330 is coupled to detectors 1340,which are coupled to A/D converter 1340. AID converter 1350 is coupledto processor 1360, which is coupled to tunable delay device 1320. Notethat polarization controller 1310 is an optional component and need notbe present in other embodiments.

The incoming optical pulses pass through polarization controller 1310 sothat the optical pulses exiting the polarization controller 1310 haveonly a single polarization such as for example, only verticalpolarization, horizontal polarization, clockwise circular polarizationor counterclockwise circular polarization. Splitter (not shown) splitsthe exiting optical pulses onto two different optical paths 1312 and1314. The optical pulses propagating on path 1314 are delayed by tunabledelay 1320 relative to the optical pulses on optical path 1312 by aperiod of time that is a fraction of the typical pulse width. Forexample, tunable delay 1320 can introduce a delay of about 1/10 of thepulse width. The optical pulses delayed by tunable delay device 1320 areoutput on optical path 1324.

The optical pulses propagating on optical paths 1312 and 1324 arereceived by optical hybrid 1330. Optical hybrid 1330 can be, forexample, a 90-degrees optical hybrid similar to those used for thesignal detection in coherent communication systems (see, for example, S.Betti, G. DeMarchis, E. Jannone “Coherent optical communicationssystems,” John Wiley and Sons, Inc., 1995). Optical hybrid 1330 can beimplemented, for example, in fiber, silica, LiNbO₃ or other materials.Although optical hybrid 1330 is shown in FIG. 13 as a 90-degrees hybridwith four outputs, other configurations are also possible.

The optical signals output from the optical hybrid 1330 are detected bydetectors 1340, which can be for example photodiodes. Although detectors1340 are balanced photodetectors, in other embodiments the detectorsneed not be balanced photodetectors. Detectors 1340 produce electricalsignals based on the received optical signals and provide thoseelectrical signals to A/D converter 1350. Although the embodiment inFIG. 13 shows four detectors 1340, other embodiments having a lesser orlarger number of detectors are possible. The digital signals produced byA/D converter 1350 are provided to processor 1360, which can be forexample a digital signal processor (DSP). For each received opticalpulse, processor 1360 can calculate a digital signal that represents theextent to which the corresponding optical pulse on optical path 1312 andthe corresponding optical pulse 1324 overlap. This digital signal can bestored in a buffer (not shown) that is accessible by processor 1360. Themeasured optical pulse corresponds to a particular delay applied bytunable delay device 1320.

The next measurement is based on a subsequently received optical pulsewhere the portion of light on optical path 1314 has a different delayapplied by tunable delay device 1320. For example, a delay on anadditional 1/10 of a typical pulse width (e.g., 2/10 for the secondmeasured optical pulse) can be applied by tunable delay device 1320.Now, the extent to which the optical pulse portion on optical path 1312and the optical pulse portion on optical path 1324 overlap will differbased on this new delay value. Processor 1360 produces another digitalsignal that represents this new overlap. This digital signal is againstored in the buffer accessible by processor 1360. This measurementprocedure is repeated (i.e., an incrementally increased delay value isapplied to subsequent optical pulses) until the optical pulse on opticalpath 1312 and the optical pulse on optical path 1324 substantially donot overlap. The effective pulse width is then calculated from thedigital signals stored in the buffer.

The pulse shape detector based on autocorrelation and described inreference to FIG. 13 can find various applications in different fields.For example, a pulse shape detector based on autocorrelation can be usedin optical communication systems regardless of the manner in whichcommunication channels are multiplexed or the manner in which data ismodulated. For example, a pulse shape detector based on autocorrelationcan be used for optical signals in a wavelength-division multiplexing(WDM) system, a time-division multiple access (TDMA) system and acode-division multiple access (CDMA) system. In addition, a pulse shapedetector based on autocorrelation can be used for optical signals havingphase-modulated data, frequency-modulated data or amplitude-modulateddata.

Moreover, the disclosed pulse shape detector based on autocorrelation isnot limited to the pulse shapes having only three or more harmonics, butcan be applied to variety of pulse shapes such as Gaussian orsuper-Gaussian pulse shapes. Because known devices that measure theautocorrelation function (i.e., the pulse width) of arbitrary shortpulses are based on second harmonics generation crystals, the disclosedpulse shape detector based on autocorrelation can have a sensitivity onthe order, more or less, of the known devices based on second harmonicsgeneration crystals.

FIG. 14 shows a pulse shape detector, according to another embodiment ofthe invention. As shown in FIG. 14, pulse shape detector 1400 includes asplitter device 1410, a dispersion device 1420 and a balanced detector1430. Splitter device 1410 is coupled to dispersion device 1420 byoptical paths 1412 and 1414. Dispersion device 1420 is coupled tobalanced detector 1430 by optical paths 1422 and 1424. In alternativeembodiments where the split optical path lengths differ (e.g., thelength of optical paths 1412 and 1422 on the one hand differ from thelength of optical paths 1414 and 1424 on the other hand), a tunabledelay device can be disposed within one of the optical paths.

Splitter device 1410 receives an optical signal and divides the opticalsignal onto two separate optical paths 1412 and 1414. The dispersiondevice 1420 introduces a dispersion having one sign onto optical path1422 and the same amount of dispersion but with the opposite sign ontooptical path 1424. In this embodiment, the dispersion device 1420 can bebased on, for example, a chirped fiber Bragg grating. FIG. 15 shows apulse shape detector with single chirped fiber Bragg grating, accordingto an embodiment of the invention.

As shown in FIG. 15, pulse shape detector 1500 includes tap coupler1550, splitter device 1510, dispersion device 1520, tunable delay device1530, variable optical attenuator (VOA) 1560 and balanced detector 1540.Note that tunable delay device 1530 and VOA 1560 are optional. Splitterdevice 1510 includes a 3-db coupler 1516. Dispersion device 1520includes circulator 1526, chirped fiber Bragg grating 1528 andcirculator 1529. Tunable delay device 1530 includes a variable opticaldelay (VOD) 1536. Although not explicitly shown, a low pass filter canbe added to the pulse shape detector 1500 to suppress unwanted noise.

A small portion of the distorted optical pulse (i.e., the receivedoptical signal after being transmitted through the communication link)is separated for the pulse shape measurement. This small optical portionof the distorted pulse is separated, for example, by tap coupler 1550,which can be for example a 10:90 coupler. The remaining portion of thedistorted optical pulse can be provided to the receiver for detectingthe modulated data. The 3-dB coupler 1516 of splitter device 1510 splitsthe light onto two optical paths 1512 and 1514.

Dispersion device 1520 then introduces a certain amount of dispersioninto the optical signals on each of these optical paths. Morespecifically, chirped fiber Bragg grating 1528 with circulator 1526introduces a certain amount of dispersion onto the optical pulsespropagating on optical path 1514. Similarly, chirped fiber Bragg gratingwith circulator 1529 introduces an equal amount of dispersion, but withan opposite sign, onto the optical pulses propagating on optical path1512. In this embodiment, the optical signals from both optical paths1514 and 1512 are reflected by the same chirped fiber Bragg grating1528. Because these optical signals are reflected from the oppositesides of the grating 1528, the sign of the introduced chirp is oppositefor the pulses from optical path 1514 and the pulses from optical path1512. The amount of chirp introduced is the same for both optical paths1514 and 1512.

In an alternative embodiment, the same chirp can be introduced by twoindependent chirped fiber Bragg gratings each being a piece of fiberwith the required dispersion characteristics. In this alternativeembodiment, one chirped fiber Bragg grating can be coupled to oneoptical path and the other chirped fiber Bragg grating can be coupled tothe other optical path.

Returning to FIG. 15, dispersion device 1520 provides the opticalsignals onto optical paths 1524 and 1522. VOA 1536 is coupled to opticalpath 1522 and is configured to vary the optical power of optical signalspropagating on optical path 1522. By varying the optical power ofoptical signals propagating on optical path 1522, the optical power ofthe optical signals can be balanced before being received at balanceddetector 1540. VOD 1536 is coupled to optical path 1524 and provides avariable optical delay for synchronizing the optical signals received atthe detector so that an incremental delay is applied as described inFIG. 14. The bandwidth of the balanced detector can be, for example,close to the symbol rate of the data represented by the received opticalsignals.

FIG. 16 shows a pulse shape detector with single chirped fiber Bragggrating, according to another embodiment of the invention. As shown inFIG. 16, pulse shape detector 1600 includes tap coupler 1650, splitterdevice 1610, dispersion device 1620, tunable delay device 1630, VOA 1660and balanced detector 1640. Note that tunable delay device 1630 and VOA1660 are optional. Splitter device 1610 includes a 3-db coupler 1616.Dispersion device 1620 includes circulator 1626, chirped fiber Bragggrating 1628 and circulator 1629. Tunable delay device 1630 includes avariable optical delay (VOD) 1636. Balanced detector 1640 includesphotodetectors 1641 and 1644, bandpass filters 1642 and 1645, Schottkydiodes 1643 and 1646, and comparator 1647. Although not explicitlyshown, a low pass filter can be added to the pulse shape detector 1600to suppress unwanted noise.

Pulse shape detector 1600 is similar to pulse shape detector 1500 shownin FIG. 15 except that balanced detector 1640 includes twophotodetectors, one on each of the optical paths. More specifically,photodetector 1641 is coupled to VOD 1636 and coupled in series tobandpass filter 1642 and Schottky diode 1643. Similarly, photodetector1644 is coupled to VOA 1660 and coupled in series to bandpass filter1645 and Schottky diode 1646. Both Schottky diodes 1643 and 1646 arecoupled to comparator 1647.

The pulse shape detectors shown in FIGS. 14 through 16 can provide anerror signal that indicates the amount of correction that thetransmitter can subsequently apply to send an optical pulse having asubstantially optimal pulse shape. In other words, the signal output bythe balanced detector can be used as an error signal at the receiverside of the communication link. If the pulse generator at thetransmitter side forms the pre-chirped pulse in such a manner that thepulse has spectrally symmetric shape after transmission through fiber(i.e., communication link), both signals input into the balanceddetector will have the same value. In such a case, correction at is notneeded. However, if the signals input into the balanced detector havedifferent values, a corresponding correction is can be provided via theerror signal to the transmitter (e.g., to computer 130, which controlsadaptive phase and attenuation control 120 and pulse generator 110 asshown in FIG. 1) so that a subsequently sent optical pulse has asubstantially optimal pulse shape.

In one embodiment, the computer at the transmitter (e.g., computer 130shown in FIG. 1) can analyzes the output signal from the pulse shapedetector at the receiver, and adaptively tune the parameters of opticalpulse generator at the transmitter side to correct the pulse shape. Thiscorrection of the pulse shape can overcome distortions to thetransmitted pulse caused by chromatic dispersion, nonlinear effects andinter-channel interference. The result of the work of such software isshown in FIG. 17. FIG. 17 a shows the received original pulse train withdistorted pulse shapes (i.e., without any correction). FIG. 15 b showsthe pulse train after several iterations for pulse reconstruction. FIG.15 c shows the pulse train after even more iterations. FIG. 15 d showsthe reconstructed pulse train, which substantially coincides in shapewith initial one. The pulse shapes were substantially completelyreconstructed as a result of the application of correction algorithm.

FIG. 18 illustrates a flowchart of the method for adaptive pulseshaping, according to an embodiment of the invention. This method can beimplemented, for example, by computer 130 and adaptive phase andattenuation control 120, shown in FIG. 1. The method is based on themaximizing of the power of the optical signal portion having thespectral component with frequency F. Maximal magnitude of the signal atfrequency F means that both (f+F) and (f−F) spectral components are inphase with the signal at main frequency f. For condition to besatisfied, the chirp introduced by the modulator at the pulse generatorat the transmitter is just correct to compensate the dispersion of thefiber, and no phase shift exists between the signal portion having thespectral component f and the signal portion having the spectralcomponents (f+F) and (f−F) at the receiver point.

This dithering method can be implemented, for example, on amicroprocessor or personal computer (e.g., computer 130 shown in FIG. 1)to control the pulse generation by maximizing the voltage output of thepulse shape detector (e.g., pulse shape detector 20). The analog voltageoutput of the power detector can be sampled and quantified to digitalsignal by an A/D converter (not shown). The computer (or microprocessor,e.g., computer 130) can then maximize this feedback signal by thedithering method depicted in FIG. 18. The D/A converter (not shown)converts the digital output signal from this method to an analog signal,which controls the phase modulator (e.g., within pulse generator 110).

As shown in FIG. 18, at step 1800, a center voltage, V₀, and a dithervoltage, ΔV, are initialized. At step 1805, a voltage, V_(1′), is set tothe center voltage, V₀, minus the dither voltage, ΔV. At step 1810, thetesting signal with V_(1′) is sent to the phase modulator of the pulsegenerator (e.g., modulator 732 of pulse generator 700 shown in FIG. 7).At step 1815, the optical pulse based on the testing signal is received(for example, at pulse shape detector 20 as shown in FIG. 1). At step1820, the voltage V_(in) of the modulation signal is detected. At step1825, the obtained value V_(in) is saved in V₁ buffer.

At step 1830, a voltage, V_(2′), is set to the center voltage, V₀. Atstep 1835, the testing signal with V_(2′) is sent to the phase modulatorof the pulse generator (e.g., modulator 732 of pulse generator 700 shownin FIG. 7). At step 1840, the optical pulse based on the testing signalis received (for example, at pulse shape detector 20 as shown in FIG.1). At step 1845, the voltage V_(in) of the modulation signal isdetected. At step 1850, the obtained value V_(in) is saved in V₂ buffer.

At step 1855, a voltage, V_(3′), is set to the center voltage, V₀, plusthe dither voltage, ΔV. At step 1860, the testing signal with V_(3′) issent to the phase modulator of the pulse generator (e.g., modulator 732of pulse generator 700 shown in FIG. 7). At step 1865, the optical pulsebased on the testing signal is received (for example, at pulse shapedetector 20 as shown in FIG. 1). At step 1870, the voltage V_(in) of themodulation signal is detected. At step 1875, the obtained value V_(in)is saved in V₃ buffer.

At step 1880, a determination is made as to which voltage V₁, V₂ or V₃is the maximum. If V₁ is the maximum of V₁, V₂ and V₃, then the centervoltage, V₀, will be decreased by the dither voltage, ΔV. If V₂ is themaximum of V₁, V₂ and V₃, then the center voltage, V₀ remains with thesame value. If V₃ is the maximum of V₁, V₂ and V₃, then the centervoltage, V₀, will be increased by the dither voltage, ΔV.

FIG. 19 shows the test setup for measuring pulse propagation in a linkwith significant dispersion using pulse shape correction, according toan embodiment of the invention. More specifically, the test setup 1900includes a distributed feedback (DFB) laser 1905, pulse generator 1910,signal generator 1915, temperature controller 1920, AMZI filter 1925,phase modulator 1930, VOA 1935, coupler 1940, dispersive fiber 1945,erbium-doped fiber amplifier (EDFA) 1950, filter 1955 andoscilloscope/OSA 1960. As shown in FIG. 19, the pulse generator is basedon the embodiment shown in FIG. 7. The test setup 1900 allows thecomparison of the propagation of pulse-shape-corrected pulses andnon-pulse-shape-corrected pulses.

DFB laser 1905 can provide an optical signal having an optical carrier(f_(o)) to pulse generator 1910. Pulse generator 1910 can be based on,for example, the embodiment shown in FIG. 7. Pulse generator 1910 caninclude, for example, a MZ modulator driven by a 6.25 GHz sinusoidalwave that generates two 6.25 GHz tones (.+−.F) around 1546.9 nm of theoptical carrier (f_(o)) received from DFB laser 1905. Theamplitude-modulated signal produced by pulse generator 1910 is then fedto an AMZI filter 1925, which can have, for example, a FSR of 12.5 GHz.The outputs of AMZI filter 1925 are sent to coupler 1940 (e.g., a 50/50coupler) and to phase modulator 1930, which is coupled in turn to VOA1935. AMZI 1925, phase modulator 1930, VOA 1935 and coupler 1940 havepolarization maintaining (PM) fibers.

The output of coupler 1940 represents the output of a transmitter and isdirected to dispersive fiber 1945 (e.g., having about −850 ps/nmdispersion). The output from dispersion fiber 1945 is amplified by EDFA1950 and filtered by filter 1955 before being monitored by monitor 1960.Monitor 1960 can be for example an oscilloscope and an optical spectrumanalyzer (OSA).

FIGS. 20 a and 20 b show an example of the output pulse waveform on anoscilloscope based on the test setup of FIG. 19. FIGS. 21 a and 21 bshow an example of the output pulse spectrum on an OSA corresponding tothe output pulse waveform shown in FIGS. 20 a and 20 b.

As shown in FIG. 20 a, signal 2000 is the pulse output of the pulsegenerator shown at 6.25 GHz with a contrast ratio of 5.4 dB. Thecarrier-to-sideband ratio is about 15.7 dB as can be seen in FIG. 21 a.Signal 2010 is the distorted pulse after propagating through dispersionfiber 1945. The contrast ratio of the distorted pulse is reduced to 2.9dB (down from 5.4 dB for the undistorted pulse (signal 2000) transmittedfrom the pulse generator 1910).

FIG. 20 b shows the output pulse waveforms after being corrected. Signal2020 is the output pulse waveform output from the pulse generator 1910based on a pulse shaping correction. Signal 2030 is the output pulsewaveform of an optical pulse having undergone a pulse-shaping correctionand after propagating through dispersive fiber 1945. As FIG. 20 b shows,the pulse that underwent a pulse-shaping correction is greatly improvedover the pulse without pulse-shaping correction. More specifically, thepulse that underwent a pulse-shaping correction has a much highercontrast ratio even after propagating through the dispersive fiber 1945.The carrier-to-sideband ratio of the pulse output from the transmitteris 8.8 dB as shown in FIG. 21 b. As these experimental results show, thewaveform and spectrum for the shaped pulses are subjected to much lessdistortions due to the link dispersion.

The inventors conducted another experiment relating to the operation ofthe method for the adaptive pulse-shape correction described above inreference to FIG. 17. FIG. 22 shows a test setup for testing the methodfor adaptive pulse-shape correction described in connection with FIG.17. Similar to the test setup shown in FIG. 19, test setup 2200 includesa distributed feedback (DFB) laser 2205, pulse generator 2210, signalgenerator 2215, temperature controller 2220, AMZI filter 2225, phasemodulator 2230, VOA 2235, coupler 2240, dispersive fiber 2245, EDFA2250, photodetector 2255, filter 2260, RF detector 2265, A/D converter2270, computer 2275 and D/A converter 2280.

The test setup shown in FIG. 22 differs from the test setup shown in 19in the signal detection portion and in the closed loop to correct thepulse pre-chirp. After EDFA 2250, the received signal is detected byphotodetector 2255. The tone 6.25 GHz (corresponding to the 6.25 GHzsignal added by pulse generator 2210) is obtained from the outputelectrical signal after using the filter 2260. The intensity of the toneis measured by RF detector 2265. This signal is converted into digitalby AID converter 2270, and digital signal processing is performed bycomputer 2275. The resulting corrected signal is converted back toanalog form by D/A converter 2280 and applied to phase modulator 2230 tocorrect the pulse shape. The dithering algorithm described in FIG. 18can be applied. As shown in FIG. 20 b, the pulse-shape-corrected pulses(signal 2030) maintain their shape in time when adaptive shapecorrection is applied. In the absence of the adaptive shape correction,the pulse shape “breathes” in time in an undesired manner.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

1. A method for generating optical pulses for optical communicationsusing a transmission link, comprising: receiving, for a first channel,an optical signal and a first clock signal, the first clock signal beingassociated with a predetermined pulse shape, the predetermined pulseshape being selected to maximize Q-factor of the signal transmitted viathe transmission link; and modifying the optical signal based on thefirst clock signal to produce an optical pulse having the predeterminedpulse shape, wherein: the optical signal is a continuous-wave (CW)signal; and the predetermined pulse shape being a quasi-return-to-zero(quasi-RZ) pulse shape, wherein the signal intensity is essentiallyabove zero between the pulses.
 2. The method of claim 1, wherein: theclock signal includes a swing voltage and a voltage bias, the swingvoltage and the voltage bias being selected to substantially maximize aQ-factor associated with the predetermine pulse shape and thetransmission link.
 3. The method of claim 1, further comprising:receiving, for the first channel, a second optical signal and a thirdclock signal, the second clock signal being based on an error signal,the error signal based on a transmission of the optical pulse over thetransmission link; and modifying the second optical signal based on thethird clock signal to produce a corrected optical pulse having acorrected pulse shape, the corrected pulse shape being associated withthe transmission characteristic of the transmission link at a timesubsequent to the predetermined pulse shape associated with the firstclock signal.
 4. The method of claim 1, further comprising: phasemodulating the optical signal or the optical pulse with data usingphase-shift keying (PSK), the first channel being a wavelength-divisionmultiplexed (WDM) channel from a plurality of WDM channels, each WDMchannel from the plurality of WDM channels being associated with its ownmodulation signal.
 5. The method of claim 1, further comprising: phasemodulating the optical signal or the optical pulse with data usingdifferential phase-shift keying (DPSK), the first channel being awavelength-division multiplexed (WDM) channel from a plurality of WDMchannels, each WDM channel from the plurality of WDM channels beingassociated with its own modulation signal.
 6. The method of claim 1,further comprising: amplitude modulating the optical signal or theoptical pulse with data using on-off keying (OOK), the first channelbeing a wavelength-division multiplexed (WDM) channel from a pluralityof WDM channels, each WDM channel from the plurality of WDM channelsbeing associated with its own modulation signal.
 7. The method of claim1, further comprising: time-division multiplexing the optical pulse orthe optical signal for the first channel with an optical pulse or anoptical signal for a second channel, the first channel and the secondchannel from a plurality of optical-time-division multiplexed (OTDM)channels.
 8. A method for generating optical pulses for opticalcommunications using a transmission link, comprising: receiving, for afirst channel, an optical signal and a first clock signal, the firstclock signal being associated with a predetermined pulse shape, thepredetermined pulse shape being selected to maximize Q-factor of thesignal transmitted via the transmission link; and modifying the opticalsignal based on the first clock signal to produce an optical pulsehaving the predetermined pulse shape, wherein: the optical signal is acontinuous-wave (CW) signal; and the predetermined pulse shape being aquasi-return-to-zero (quasi-RZ) pulse shape, comprising: forming theoptical signal as a continuous wave and having a first spectralcomponent, the optical signal being modified so that the optical pulsehas the first spectral component, a second spectral component and athird spectral component, the second spectral component and the thirdspectral component of the optical pulse being associated with the secondclock signal.
 9. The method of claim 8, wherein the modifying includes:splitting the optical signal onto a first optical path and a secondoptical path, the first spectral component of the optical signal beingassociated with the first optical path, the second spectral componentand the third spectral component of the optical signal being associatedwith the second optical path; modifying a phase of the first spectralcomponent of the optical signal on the first optical path based on afirst phase adjustment signal; and combining the first spectralcomponent of the optical signal associated with the first optical pathwith the second spectral component and the third spectral component ofthe optical signal associated with the second optical path, wherein thefirst phase adjustment signal being associated with the first clocksignal.
 10. The method of claim 8, wherein the modifying includes:splitting the optical signal onto a first optical path and a secondoptical path, the first spectral component of the optical signal beingassociated with the first optical path, the second spectral componentand the third spectral component of the optical signal being associatedwith the second optical path; modifying a phase of the second spectralcomponent and the third spectral component of the optical signal on thesecond optical path based on a second phase adjustment signal; andcombining the first spectral component of the optical signal associatedwith the first optical path with the second spectral component and thethird spectral component of the optical signal associated with thesecond optical path, wherein the second phase adjustment signal beingassociated with the first clock signal.
 11. The method of claim 10,further comprising: modifying a phase of the first spectral component ofthe optical signal to produce an optical pulse having the predeterminedpulse shape.
 12. A method of the claim 8, further comprising: detectinga signal based on an optical pulse having the first spectral component,the second spectral component and the third spectral component, thesecond spectral component and the third spectral component being basedon the second clock frequency; filtering the detected signal to producea first signal associated with the second clock frequency; and detectingan amplitude of the first signal.
 13. The method of claim 12, wherein:the filtering is performed over a bandpass spectral response, the secondclock frequency being within the spectral response of the bandpassfilter.
 14. The method of claim 12, further comprising: receiving aplurality of optical pulses including a training optical pulse and theoptical pulse modulated with data.
 15. The method of claim 12, wherein:the second clock frequency is a microwave frequency; the first spectralcomponent of the optical pulse has an optical frequency; the secondspectral component of the optical pulse has its own frequencycorresponding to a sum of the optical frequency and the microwavefrequency; and the third spectral component of the optical pulse has itsown frequency corresponding to a difference of the optical frequency andthe microwave frequency.
 16. The method of claim 12, wherein: theamplitude of the first signal is associated with a pulse width of anoptical pulse received from the transmission link; and an error signalbeing associated with a difference between the pulse width of theoptical pulse and a predetermined pulse width, the predetermined pulsewidth being based on a transmission characteristic of the transmissionlink.
 17. A method of claim 12, further comprising: sending a pluralityof testing signals each being associated with its own dithered valuefrom a plurality of dithered values, each dithered value from theplurality of dither values being associated with at least one from acenter value and an offset value; receiving a plurality of opticalpulses each being uniquely associated with an testing signal from theplurality of testing signals; detecting a plurality of modulationsignals based on the plurality of optical pulses, each modulation signalfrom the plurality of modulation signals having its own amplitude and aspectral component with a modulation frequency; and calculating a newcenter value based on the amplitude of modulation signals.
 18. Themethod of claim 17, wherein: the plurality of testing signals includes afirst testing signal, a second testing signal and a third testingsignal, the sending includes: sending the first testing signal, thefirst testing signal being associated with a difference of a centervalue and an offset value; sending the second testing signal, the secondtesting signal being associated with the center value; sending the thirdtesting signal, the third testing signal being associated with a sum ofa center value and an offset value.
 19. The method of claim 17, whereinthe receiving includes: receiving an optical pulse based on the firsttesting signal, the optical pulse for the first testing signal beingassociated with its own amplitude; receiving an optical pulse based onthe second testing signal, the optical pulse for the second testingsignal being associated with its own amplitude; and receiving an opticalpulse based on the third testing signal, the optical pulse for the thirdtesting signal being associated with its own amplitude.
 20. The methodof claim 17, wherein: the new center value is calculated by selecting amaximum amplitude from the amplitude of modulation signal for the firsttesting signal, the amplitude of the modulation signal for the secondtesting signal and the amplitude of the modulation signal for the thirdtesting signal.
 21. An apparatus for an optical pulse shape formationfrom a continuous optical wave having an optical frequency f,comprising: an optical modulator driven by a second clock signal with afrequency F, a demultiplexer connected to an output of the opticalmodulator, the demultiplexer splitting the received light of frequenciesf+F, f, f−F into different waveguides, a phase shifter introducing aphase shift in at least one of the spectral components f+F, f or f−F,and a multiplexer combining light of all three spectral components, adata modulator for data embedding in the optical pulse for transmissionin a communication link.
 22. The apparatus of claim 21, wherein thedemultiplexer is an asymmetric Mach-Zender interferometer.
 23. Theapparatus of claim 21, wherein the multiplexer is an asymmetricMach-Zender interferometer.
 24. The apparatus of claim 21, wherein themultiplexer is the demultiplexer operating in backward lighttransmission mode.
 25. The apparatus of claim 21, wherein the frequencyF is microwave.
 26. The apparatus of claim 21, wherein the datamodulator modulates the optical pulse with data using phase-shift keying(PSK).
 27. The apparatus of claim 21, wherein the data modulatormodulates the optical pulse with data using differential phase-shiftkeying (DPSK).
 28. The apparatus of claim 21, wherein the data modulatormodulates the optical pulse with data using on-off keying (OOK).